The present invention relates to methods and devices for removing hydrogen sulfide from liquid sulfur such as the one produced by a Claus process.
The Claus process is a well known process for producing elemental sulfur by reacting hydrogen sulfide and sulfur dioxide. Typically, hydrogen sulfide in refinery product gases is partially combusted to produce sulfur dioxide, which then reacts with the unburned hydrogen sulfide to yield sulfur and water. The sulfide is condensed and recovered. Suitable catalysts may also be used to promote the sulfur production.
The basic chemistry of the Claus process is as follows:
H2S+{fraction (3/2)}O2xe2x86x92SO2+H2O
2H2S+SO2xe2x86x92xe2x85x9cS8+2H2O
Hydrogen sulfide is soluble in liquid sulfur. The dissolved hydrogen sulfide may react with the sulfur diradical chain species to form hydrogen polysulfides, as follows:
H2S+xc2x7Sxxe2x88x921xc2x7xe2x86x92H2Sx
The conversion rate of hydrogen polysulfide back to hydrogen sulfide is generally very low.
One problem associated with the Claus process is the presence in the produced liquid sulfur of dissolved hydrogen sulfide. The hydrogen sulfide contaminates the products and may pose serious hazards. For example, hydrogen sulfide may create nuisance odors in the vicinity of liquid sulfur, and may also reach toxic levels when loading and unloading the sulfur. Also, when storing liquid sulfur in tanks or other vessels, a threat exists that the lower explosive limit of hydrogen sulfide may be reached.
Typically, sulfur produced from a Claus sulfur plant may contain from about 150 to about 450 wppm (weight parts per million) of hydrogen sulfide which may evolve with time in the storage facilities such as pits, tanks, tank trucks, and/or tank cars and accumulate in the vapor phase creating unsafe conditions. For instance, hydrogen sulfide can escape out of the storage device resulting in a release to the atmosphere. Hydrogen sulfide in the vapor phase of a storage device may build up to a level exceeding its lower explosive limit and cause an explosion. Sulfur storage explosions have occurred in the industry.
To eliminate these and other problems, a number of methods have been suggested or developed that remove hydrogen sulfide from liquid sulfur. Release of dissolved hydrogen sulfide has been carried out by agitating the liquid sulfur by various means, such as by mechanical agitating means, spraying means, by bubbling air through the liquid sulfur, and by circulating the sulfur over a stripping column. The released hydrogen sulfide is often removed from the pit gas space by a sweep gas, which typically may be any suitable inert gas such as air, Claus tail gas or nitrogen.
For example, U.S. Pat. No. 5,080,695 (""695) issued on Jan. 14, 1992 to Kassarjian, describes a method of removing hydrogen sulfide from liquid sulfur wherein the liquid sulfur is caused to flow continuously through a seal pot where it is contacted by a counter-flowing inert gas, thereby stripping hydrogen sulfide from the sulfur. The inert gas which is typically, nitrogen flows back to the Claus process. One problem with the method of the ""695 patent is that selection of degassing agent is restricted to inert gases which have been demonstrated to be less effective degassing agents than oxygen containing gas streams, such as air. Since the spent gas is returned to the process in the ""695 patent, an oxygen containing gas, such as air, could not be used because of safety concerns.
Presently, most industrial facilities remove hydrogen sulfide from the liquid sulfur by injecting a gas, such as air, nitrogen and steam in the pit where liquid sulfur is stored. Some have installed elaborately designed nozzles inside the pit for dispersing the gas into the pit.
However, installation or replacement of air sparging or mixing equipment in a storage device such as a pit requires emptying out the entire is sulfur inventory. This typically requires shutting down the entire sulfur unit for several days because of the highly corrosive, and pyrophoric environment in the pit. The pit should be depleted of any sulfur and made safe before human personnel may enter and make the installation. Other problems exist with known methods for reducing the amount of hydrogen sulfide in liquid sulfur.
The present invention offers an improved, cost-effective method and apparatus to remove a substantial quantity of hydrogen sulfide from liquid sulfur. The method is particularly suitable for removing hydrogen sulfide from liquid sulfur produced from a Claus process.
One aspect of the present invention relates to an improved method for removing hydrogen sulfide from liquid sulfur. The method comprises introducing liquid sulfur containing hydrogen sulfide in an upstream portion of a conduit. The conduit has a fluid outlet in a downstream portion thereof which is located within a lower portion of a first vessel. The liquid sulfur is caused to flow through the fluid outlet of the conduit into the first vessel and then up an annulus space that is formed between the exterior of the conduit and the interior of the first vessel. The liquid sulfur overflows through a first vessel outlet positioned at an upper portion of the first vessel into a second vessel. The second vessel is connected to a liquid sulfur storage device such as a pit that operates typically at atmospheric pressure. The method further includes introducing air or enriched oxygen air into the liquid sulfur located in the annulus formed between the exterior of the conduit and the interior surface of the first vessel at a point that lies between the conduit outlet and the first vessel outlet. The air is caused to flow concurrently to the direction of flow of the liquid sulfur in the annulus and out of the first vessel outlet into the second vessel thereby removing hydrogen sulfide from the liquid sulfur in said annulus space.
One advantage of the present inventive method is that air and/or enriched oxygen air can be used instead of the more expensive and less effective inert gases that are typically used because of safety concerns. Air and/or enriched oxygen air can be used because it is not allowed to go upstream to the Claus process. More specifically according to the present inventive method, the air travels concurrently with the upward flow of liquid sulfur in the annulus space formed between the conduit and the first vessel. Thus, the air is transferred to a sulfur storage vessel or pit that is typically open to the atmosphere instead of going upstream to the Claus process.
Another advantage of the present inventive method is that it does not require taking out of service a sulfur storage device to install or to maintain the equipment necessary to practice the method, since air is injected into a separate vessel such as a seal pot of a Claus process.
Yet another advantage of the present invention is that the method can be used with a catalyst to promote degassing. These and other advantages of the present invention will become more apparent to persons skilled in this art from the following detailed description of the invention in conjunction with the following figures.